Natural killer (NK) cells comprise a significant proportion of joint-infiltrating cells but their role in the pathogenesis of rheumatoid arthritis (RA) remains to be elucidated. A distinct CD56bnght NK subset that produces high levels of interferon-n (IFND) and co-activates macrophages has been found to preferentially infiltrate afflicted joints. By contrast, a CD56dim NK subset that has poor inflammatory activity but heightened cytolytic function is underrepresented in inflamed joints. However, it remains to be investigated whether these two NK subsets represent sequential stages of maturation or different developmental lineages. This is an important issue because different ontogeny may reflect distinct biological functions that influence clinical disease outcomes. We, and others have shown that VDJ recombination, the process of antigen receptor rearrangement in B cells and T cells, also occurs in human and murine NK cells. Based on analysis of recombination in NK cells in a mouse model, we hypothesize that inflammatory NK cells represent an ontogenetically distinct lineage. Using indelible DNA rearrangements as a clonal marker, we propose to determine the developmental relationship between CD56bright and CD56dim NK cells in patients with RA (Aim 1). We then take advantage of a transgenic mouse model that expresses green fluorescent protein (GFP) consequent to VDJ recombination, in order to examine the developmental kinetics and activation potential of recombination"" (GFP+) versus recombination" (GFP") NK cells (Aims 2 &3). Characterizing the developmental dynamics and functional activities of recombination* inflammatory NK cells could provide insights into the development of alternative therapies for RA.